1. Field of the Invention
The present invention relates to a scanning optical system, and more particularly, to a scanning optical system suitable in an image output portion of a laser beam printer, a copying machine, or the like.
2. Description of the Related Art
In recent years, in a scanning optical system in an image output portion of a laser beam printer, a copying machine, or the like, reduction of costs by manufacture of an imaging optical element by resin molding, enhanced speed by a surface emission type semiconductor laser (VCSEL), and higher definition by reduction of a spot size are in progress.
When an imaging optical element is manufactured by molding, a plurality of identical components is manufactured at the same time in order to improve productivity, and thus, variations in performance occur between the identical components, and in particular, variations in a direction of focus are wide.
A spot diameter is the same as the diameter of a circle of confusion, and thus, reduction of a spot size for higher definition means reduction of a width of a depth of the focus. Therefore, a slight displacement of the focus results in deviation from an allowable depth. In particular, an off-axis place is affected more by aberration than an on-axis place, and thus, in such a place, the width of the depth of the focus is further reduced.
Further, the following problem is involved in a multi-beam system for enhancing the speed. A semiconductor laser generally has a Gaussian distribution type intensity distribution in a direction perpendicular to an emission direction (far-field pattern: FFP). Depending on locations of a plurality of light emission points, the degree of vignetting of the FFP changes due to a stop arranged for controlling the spot diameter on a surface to be scanned. The larger a distance from an optical axis to the light emission point is, the greater the influence is, and thus, the intensity distribution of light beams from the light emission points which pass through the stop becomes asymmetrical. Increased asymmetry results in an increased spot diameter and a reduced width of a depth of a focus. In particular, a surface emission type semiconductor laser tends to have a narrower FFP compared with that of a related-art edge emission type semiconductor laser, and thus, the intensity distribution is more liable to become asymmetrical.
Japanese Patent Application Laid-Open No. H11-311748 discloses, as a method of maintaining the spot diameter and securing the width of the depth of the focus, a method in which, with regard to a single light emission point, a size of an aperture of a stop provided in a scanning optical system is controlled so as to be at a predetermined ratio with respect to a width of incident light beams (1/e2 intensity slice of FFP).
Japanese Patent No. 3170798 discloses a method in which, with regard to a two-dimensionally arranged multi-beam, the relationship among a location of a light emission point, a location of a stop, and a focal length of a collimator is controlled so that a ratio of a size of an aperture of the stop to a width of incident light beams is a predetermined ratio. In the technology disclosed in Japanese Patent No. 3170798, by equalizing the degree of vignetting of the light beam by the stop among beams in the multi-beam, the spot diameter and the width of the depth of the focus can be the same among the beams in the multi-beam. Further, when the light emission points of a related-art multi-beam system such as one having two to four beams are in proximity to the axis, a range in which the stop can be located is relatively wide, and thus, the stop can be provided at an arbitrary location.
The technology disclosed in Japanese Patent Application Laid-Open No. H11-311748 is on the precondition that there is a single light emission point and that a light beam which passes through the stop has a symmetrical intensity distribution, and does not give any technical review of an optical system in which a light beam which passes through the stop has an asymmetrical intensity distribution such as one in which a light emission point is outside the multi-beam.
In recent years, for the purpose of enhancing the speed, a multi-beam including eight or more beams is sometimes used. In a system in which the location of the light emission point is away from the axis, a structure disclosed in Japanese Patent No. 3170798 requires the stop to be provided in proximity to a focal point on an image side of the collimator. In a scanning optical system, in the case of the multi-beam, an angle of a ray of light which enters a light deflector affects displacement of a writing-out location among light emission points due to displacement of the focus of a scanning system, and thus, it is necessary that the stop be provided in proximity to the light deflector. Therefore, in the case of the multi-beam, because the location of the stop is limited when the structure disclosed in Japanese Patent No. 3170798 is employed, it is difficult to provide the stop in proximity to the light deflector.